Recently, digital receivers, such as mobile telephone sets, are becoming increasingly popular in the mobile communication field. In such a digital receiver, the average power of received signals usually has a D-range (Dynamic-range) which is higher than 70 dB. Temporarily, the received signals are undesirably changed in level by fading. For stable communication, it is important to control the level of the received signals, and therefore, most digital receivers are provided with automatic gain control apparatuses to maintain the received signal at an optimum level.
A conventional automatic gain control apparatus includes a gain-controllable amplifier, which amplifies an analog input signal at a gain level determined in accordance with a gain control signal. The gain-controllable amplifier is connected at an output terminal to an A/D (Analog-to-Digital) converter, which converts the amplified analog signal into a digital signal. The digital signal is supplied to a demodulation circuit, which recovers the data from the digital signal. The gain control signal is generated based on the difference between the average power of the digital signal and a reference value, which is nominally determined in advance. In general, the reference value is determined to have a favorable level so that the analog signal entered into the A/D converter has a maximum amplitude which corresponds to the maximum permissible level of the A/D converter. To determine the reference value in such a manner allows the A/D converter to have an optimum S/N (Signal to Noise) ratio and to output a good shape of digital signal. Namely, the levels of the reference value and the gain control signal are determined so as to obtain a high S/N ratio and a good shape of digital signal, from which modulated data may be demodulated accurately.
According to the earlier described conventional automatic gain control apparatus, however, there is a disadvantage in that the amplitude of the analog input signal is not always controlled at a favorable gain level because of a direct-current offset. In more detail, a direct-current offset may undesirably be added to the analog input signal at the gain-controllable amplifier or the A/D converter. When the direct-current offset is added to the analog input signal, the signal may have a maximum amplitude which is over the maximum permissible level of the A/D converter. If the A/D converter is supplied with an over level of input signal, the digital output signal has a distorted waveform. Such a distorted wave form of digital signal disturbs accurate demodulation in the demodulation circuit. For producing a good shape of digital signal, if the direct-current offset is nominally taken away from the reference value, the maximum amplitude of the analog input signal may become much lower than the maximum permissible level of the A/D converter, and therefore, the S/N ratio gets worse.